If
Restrict users to the explicitly permitted list of snap-ins
is enabled, users cannot use any snap-in except those explicitly permitted. To explicitly permit use of this snap-in, enable this policy. If this policy is not configured (or disabled), this snap-in is prohibited.

If
Restrict users to the explicitly permitted list of snap-ins
is disabled or not configured, users can use any snap-in except those explicitly prohibited. To explicitly prohibit use of this snap-in, disable this policy. If this policy is not configured (or enabled), the snap-in is permitted.

When a snap-in is prohibited, it does not appear in the Add/Remove Snap-in window in MMC. Also, when a user opens a console file that includes a prohibited snap-in, the console file opens, but the prohibited snap-in does not appear.